Strange 'pimples'

Anyone who has ever taken a look at Sycamore leaves, will undoubtedly have seen lots of small, red pimples on the surface of some of them.

I have seen these many times in the past and always thought they were formed by the action of gall wasps. well, I was wrong. They are a type of gall, but not produced by insects at all. They are in fact the result of infestation by a mite called Aceria macrorhynchus. The name is much bigger than the mite!

As you can see from the scale on the picture, these mites are indeed minute! At least ten of them would comfortably sit on the head of a pin. The female mites spend the winter in crevices in the bark, but emerge to feed on the young leaves as they unfurl. Galls arise where the mites have been feeding, and one mite can cause many galls. Later, the mites lay their eggs in some of the galls, and their developing larvae feed on the tissue inside, emerging in the autumn.